A number of impact sensors have heretofore been proposed for activating vehicle restraint systems and other similar applications. For example, U.S. Pat. No. 4,329,549 discloses a sensor in which a spherical ball is biased toward one end of a cavity by a permanent magnet positioned externally of the cavity. Severe deceleration of a vehicle, on which the sensor is mounted and appropriately oriented, exerts sufficient force on the ball to overcome the force of attraction to the magnet, and to propel the ball longitudinally through the cavity against a pair of switch contacts. The switch contacts are connected to suitable devices for inflating an air bag or the like to prevent impact of a vehicle occupant against the steering wheel or dashboard.
A disadvantage of this design is that closely controlled tolerances are required between the ball and the surrounding cavity, which are difficult and expensive to control in manufacture. Furthermore, the ball must be sufficiently large to generate reliable contact force against the switch elements, necessitating use of a large magnet and consequently decreasing responsiveness of the sensor to impact forces. Weight of the ball may be reduced by plating the switch contacts with gold or the like, increasing the cost of manufacture. Dust particles and the like decrease reliability of the mechanical switch contacts. Additionally, the sensor disclosed in this patent is not well adapted to respond to impacts from all directions, necessitating use of more than one sensor in automotive applications.
U.S. Pat. No. 4,827,091 discloses an impact sensor in which a permanent magnet is longitudinally movable within a cavity of a non-magnetic body. The magnet is biased toward one end of the cavity by magnetic attraction to a ring positioned externally of the cavity. One or more coils surround the cavity for sensing movement of the magnet therewithin. In addition, electrical switch contacts may be positioned at the opposing end of the cavity for abutting electrical and mechanical engagement with the magnet.
U.S. Pat. No. 4,484,041 discloses an impact sensor in which a permanent magnet is movably mounted within a cavity of a non-magnetic body, and normally held by magnet attraction against a magnetically permeable element that closes one end of the cavity. Application of acceleration forces on the body sufficient to overcome the force of magnetic attraction propels the magnet against a spring at the opposing end of the cavity, which returns the magnet to its normal position when the acceleration forces are removed. A reed switch is positioned adjacent to the cavity to detect motion of the magnet against the spring.
U.S. Pat. No. 4,639,563 discloses a sensor in which a pair of magnets are movably mounted within the cavity of a non-magnetic body and oriented such that like poles on the magnets are opposed to each other and hold the magnets against opposing ends of the cavity. A reed switch is positioned adjacent to the cavity and generates an output signal when acceleration forces on either magnet are sufficient to overcome the force of magnetic repulsion and move the magnet toward the center of the cavity.
A general object of the present invention is to provide an acceleration sensor that has particular utility as an impact sensor for activating a vehicle safety restraint system, that is compact and rugged in construction, that is reliable in operation, that activates quickly on application of vehicle impact forces, and/or that is inexpensive to manufacture. Another and more specific object of the present invention is to provide an impact sensor of the described character that embodies efficient, economical and reliable solid state technology. Yet another object of the present invention is to provide a vehicle restraint system that employs such a sensor.